A semiconductor device which emits light with a visible range to an ultraviolet range, i.e., a semiconductor laser or a light emitting diode has been one of the important semiconductor devices in the modern society/industrial world, since the semiconductor laser or the light emitting diode can be used for an optical information recording device (a compact disc (CD), a digital versatile disc (DVD) or a Blue-ray disc (BD)), a light source for a color display apparatus, a device for excitation of a solid-state laser, a device for processing, a device for a sensor, a device for a measuring instrument, a device for medical care, or a device for application to a white lamp.
As a semiconductor material for these optical elements, a group III-V compound such as AlGa(In)As has been used in an infrared device having a 780 nm, 808 nm, 860 nm, 915 nm or 980 nm band up to today. An AlGaInP III-V compound semiconductor is used as a material in the red light device having a light emitting wavelength of a 600 nm band (especially, 635 to 670 nm). In addition, an AlGaInN III-V nitride semiconductor is used in a blue light device having a light emitting wavelength of a 400 nm band (especially, 400 to 480 nm). The red light device and the blue light device have come into practical use through the progress of the research and the development.
However, with regard to the semiconductor devices for emitting light with a wavelength corresponding to yellow light to green light which is about a 500 nm band and is an intermediate wavelength band between a wavelength band of red light and a wavelength band of blue light, the material exploitation has not been sufficiently performed, not to mention the research and development. For this reason, in particular, with respect to the laser diode, the performance enough to realize its practical application has not yet been realized.
The group II-VI compound semiconductor, along with the group III-V compound semiconductor, is useful as semiconductor materials for these optical devices. In general, however, since a p-type conductivity property is difficult to control, a p-type semiconductor layer of a pn junction type semiconductor device can be realized only when limited kinds of group II-VI compound semiconductors such as ZnSe are used. In the group II-VI compound semiconductors, in general, as a band gap width becomes wider, a p-type carrier concentration decreases, resulting in that any of these group II-VI compound semiconductors cannot be utilized in the pn junction type semiconductor device. For example, in the case of ZnMgSSe which is lattice matched to a GaAs substrate, a band gap width can be widened in accordance with an increase in Mg composition ratio. When the band gap width becomes equal to or larger than 3 eV, however, a p-type carrier concentration becomes a small value smaller than 1×1017 cm−3. In addition, even in the case of an MgSe/ZnSeTe superlattice which is lattice matched to the InP substrate, when the band gap width becomes equal to or larger than 2.6 eV, likewise, only a small p-type carrier concentration is obtained. The technique as described above is disclosed in a literature of H. Okuyama, Y. Kishita, T. Miyajima and A. Ishibsashi, “Epitaxial growth of p-type ZnMgSSe”, Appl. Phys. Lett., 64(7) 1994, page 904, and a literature of W. Shinozaki, I. Nomura, H. Shimbo, H. Hattori, T. Sano, Song-Bek Che, A. Kikuchi, K. Shimomura and K. Kishino, “Growth and characterization of nitrogen-doped MgSe/ZnSeTe superlattice quasi-quaternary on InP substrates and fabrication of light emitting diode”, Jpn. J. Appl. Phys., 38(4B) 1999, p. 2598.
Under such circumstances, the inventors of this application, and several research groups in Japan and other countries have paid attention to an MgxZnyCd1-x-ySe II-VI compound semiconductor which can be formed through crystal growth on an InP semiconductor substrate and which is lattice matched to the InP substrate as a candidate of a material for formation of semiconductor devices which emit light with a wavelength corresponding to yellow light to green light, and have researched and developed the MgxZnyCd1-x-ySe II-VI compound semiconductor. This technique is disclosed in a literature of N. Dai et al.: Appl. Phys. Lett., 66, 2742(1995) and a literature of T. Morita et al.: J. Electron. Mater., 25, 452 (1996). The MgxZnyCd1-x-ySe II-VI compound semiconductor has such a feature that the MgxZnyCd1-x-ySe II-VI compound semiconductor is lattice matched to InP when its composition ratios (x, y) fulfill a relationship of y=0.47−0.37x (x=0 to 0.8, y=0.47 to 0.17), and its band gap width can be controlled from 2.1 eV to 3.6 eV by changing its composition ratios from (x=0, y=0.47) to (x=0.8, y=0.17).
In addition, in the above-mentioned composition range, all the band gap widths show a direct transition type. The band gap width corresponds to a wavelength of 590 nm (orange color) to 344 nm (ultraviolet) when being converted into a wavelength. This suggests that an active layer and a cladding layer which constitute a double hetero structure as a basic structure of each of the semiconductor devices which emit light with a wavelength corresponding to yellow light to green light can be realized by only changing the composition ratios of MgxZnyCd1-x-ySe.
Actually, in the measurement of photoluminescence of MgxZnyCd1-x-ySe which is grown on an InP substrate by utilizing a molecular beam epitaxy (MBE) method, the satisfactory light emitting characteristics are obtained in which a peak wavelength ranges from 571 nm to 397 nm in the MgxZnyCd1-x-ySe materials having different composition ratios. The satisfactory light emitting characteristics are described in the literature of T. Morita et al.: J. Electron. Mater., 25, 425 (1996).
In addition, for a laser structure using MgxZnyCd1-x-ySe, laser oscillation has been reported, due to light excitation in wavelength bands corresponding to red light, green light and blue light. This report is described in a literature of L. Zeng et al.: Appl. Phys. Lett., 72, 3136 (1998).
On the other hand, the laser oscillation due to current driving for a semiconductor laser diode constituted by only MgxZnyCd1-x-ySe has not yet been reported until now. It is considered that the main cause for which the laser oscillation is not obtained is due to difficulty in p-type conductivity controlled by impurity doping into MgxZnyCd1-x-ySe.
The double hetero structure is the basic structure of the semiconductor laser diode. The active layer is sandwiched between the cladding layers each having a band gap width wider than that of the active layer. Here, it is obvious from the above-mentioned research reports that MgxZnyCd1-x-ySe has the excellent property in terms of the active layer material.
In addition, the n-type conductivity control for MgxZnyCd1-x-ySe is obtained from the doping of chlorine atoms, and an n-type carrier (electron) concentration of 1×1018 cm−3 or more has been reported. This report is described in a literature of W. Lin et al.: Appl. Phys. Lett., 84, 1432 (1998). However, with regard to a p-type conductivity control, a p-type carrier concentration of 1×1017 cm−3 or more which is required for the laser diode has not yet been reported.
Conventionally, a technique for the doping of radical nitrogen having a high energy during the crystal growth made by utilizing the molecular beam epitaxy method has been mainly adopted for the p-type conductivity control for the group II-VI compound semiconductor, especially, ZnSe or MgZnSSe. This technique is described in a literature of R. M. Park et al.: Appl. Phy. Lett., 57, 2127 (1990) and a literature of K. Ohkawa et al.: Jpn. J. Appl. Phys., 30, L152 (1991). Thus, the p-type carrier concentration of 1×1017 cm−3 or more has been reported. This report is described in the literature of H. Okuyama et al.: Appl. Phys. Lett., 64, 904 (1994).
The p-type conductivity control for MgxZnyCd1-x-ySe has been attempted by using the same technique. However, the p-type doping concentration of 3.5×1016 cm−3 in ZnCdSe having composition ratios (x=0, y=0.48) has been only reported. This report is described in a literature of K. Naniwae et al.: J. Cryst. Growth, 184/185, 450 (1998). Thus, neither of the high doping concentration of 3.5×1016 cm−3 or more nor the p-type conductivity in MgxZnyCd1-x-ySe (x>0) having a band gap wider than that of ZnCdSe has not yet been obtained.
The cause of difficulty of the high p-type doping in MgxZnyCd1-x-ySe has not yet been clarified. However, it is considered that the cause may be that at least an ideal impurity dopant which is replaced with a part of atoms arranged in the MgxZnyCd1-x-ySe crystal to be stably present in the crystal and which efficiently discharges holes with low energy, has not yet been found out, or is not present. This is an essential problem in characteristics which MgxZnyCd1-x-ySe has.
At that, for the purpose of increasing the maximum p-type carrier concentration which a cladding layer having a wide band gap can obtain, it has been proposed that a p-type cladding layer is made in the form of a superlattice structure made of a material including at least any one atom of Mg, Ca, Sc, Ti, V, Cr, Mn, Fe, Co and Ni, in a light emitting element constituted by a group II-VI compound semiconductor. However, the sufficient characteristics have not yet been obtained. This report is described in Japanese Patent Laid-open Publication No. 07-326817.